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Turbulent / Non-Turbulent Interface and Uniform Momentum Zones of High-Speed Turbulent Boundary Layers Subjected to Streamline Pressure Gradients
The turbulent/non-turbulent interface (TNTI) and uniform momentum zones (UMZ) within fully developed high-speed turbulent boundary layers subjected to streamline pressure gradients are investigated. Both favorable and adverse pressure gradients are generated using wall curvature in a Mach 4.9 freestream flow. The pressure gradient effects on the boundary layer mean properties and turbulent statics have been previously reported by [1] and [2] for FPU and APG, respectively. In the present work, the average TNTI height was shown to decrease with strong favorable pressure gradient, indicating reduced turbulence strength. The TNTI length per unit streamwise length increased for the SFPG case and slightly decreased for the APG cases. Together, the observed changes in TNTI height and length indicate that the curvature-driven pressure gradients are non-linearly affecting the TNTI by bringing it deeper into the boundary layer while simultaneously increasing the surface area of the interface for the SFPG case. Uniform momentum zones were also observed in the high-speed turbulent boundary layers. Qualitatively, the number of UMZs was seen to increase with more favorable pressure gradient even while the TNTI moved lower in the boundary layer. An outstanding question is the influence of compressibility on the structure of the TNTI and UMZs within a turbulent boundary layer. The majority of studies of TNTI and UMZs have focused on low Reynolds number, incompressible flows and as such, the quantitative conclusions drawn from those works may not apply to the present data. Further investigation into these compressibility effects are warranted and underway.
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